a. Field of the Invention
This invention relates to a speed control (travel) circuit and/or braking circuit for a series DC motor. More particularly, it relates to a series DC motor operating in the field weakening region in which the armature current is set by a pulsed control element and in which the field has a bypass valve.
b. Description of the Prior Art
In most cases, series connected direct current motors are designed so that they generate the full countervoltage at a fraction of their maximum permissible speed. In the following, this speed is called the "characteristic speed". Such motors are often operated from zero speed to the characteristic speed with the maximum permissible armature current, thus gaining maximum acceleration. In order to obtain maximum acceleration above the characteristic speed, the field strength is steadily reduced as speed is increased from the characteristic speed upward (here called the field weakening region). The field is weakened in such a manner that the armature current does not exceed the value of the maximally permissible armature current and remains constant at this value. Since the armature current and voltage of the motor are constant in the field weakening region, the power output remains constant while the delivered torque, which is determined by the field current, decreases with increasing speed. It should be noted that the degree of field weakening, i.e., the ratio of field current to armature current, must not fall below a predetermined minimum value as the speed increases further. If this minimum value is reached at a particular high speed, then the armature current is also reduced with further increases in speed. Thus, at the maximum permissible speed, neither the field current nor the armature current have reached the maximally permissible values.
Two speed control circuits having a final DC control element are known which utilize the principle of field weakening to increase speed above the characteristic speed.
One such speed control circuit has been used for some time by Siemens AG in vehicles using direct current propulsion, particularly in trolley cars and subways. It comprises a series circuit consisting of a DC cotrol element and a smoothing choke between the DC line and the DC series motor as well as a bypass valve in shunt with the smoothing choke and the DC series machine. In addition, a field weakening resistor, which can be shorted successively by a series of switching members, is connected parallel to the field winding via an electromechanical switch. If the speed is increased beyond the characteristic speed, an increasing part of the armature current is conducted through the stepwise variable, field weakening resistor. As the effective field weakening resistance is reduced, the branch current in this resistor increases and the branch current in the parallel field winding decreases. The sum of the two branch currents is always equal to the armature current, which is held at its maximum permissible value when starting at maximum torque.
The second speed control circuit, which also serves for starting, is described in the journal, "Elektrotechnik und Maschinenbau", vol. 85 (1968), No. 3, pages 110 to 117 and shown, particularly in FIG. 5. This circuit is also intended for the DC propulsion of vehicles. It has an input circuit consisting of a choke and a capacitor, which is connected to the DC supply line and, via a DC final control element, to the armature winding of the DC series machine. Here the field winding is not connected to the main current; instead, it is shunted across the armature winding in series with a first bypass diode. The field winding, in turn, is shunted by a second bypass diode. In this speed control circuit, the field weakening occurs automatically if the duty cycle of the DC control element is high. The particular DC final control element provided utilizes a main thyristor with a parallel quenching circuit. The quenching circuit consists of a quenching thyristor in series with a quenching capacitor; the series circuit of a reversing choke and a reversing diode is connected antiparallel to the quenching thyristor. In this circuit, the quenching capacitor in the DC control element must be overloaded at the end of the quenching process, i.e., an overvoltage is produced at the DC control element. The DC control element must therefore be designed at considerable cost for this overvoltage.
The DC series machine in which the armature winding and the field or exciter winding are connected in series is used for various drive purposes, especially in the electric propulsion of vehicles. If this machine is braked, it is desirable, for several reasons, to be able to set the field current flowing through the field winding at a lower level than the current flowing through the armature winding, i.e., also to operate in the field weakening region.
Another braking circuit of the type mentioned above is described in Siemens-Zeitschrift, vol. 47 (1973), no. 3 pages 155 to 159, particularly at FIG. 2. This braking circuit is intended for electric trolley propulsion. It is derived from a travel circuit by regrouping electrical components by means of switching devices. The known braking circuit is arranged for pure resistance braking operation, i.e., not for mixed regenerative and resistance braking. It is designed so that, for braking, i.e., after the individual components of the speed control circuit are regrouped, a braking current can flow in a closed circuit which is formed of the field and armature windings of the DC series motor, a smoothing choke, a DC control element which is shunted by a bypass valve in series with a braking switch and an ohmic braking resistor, a further uncontrolled bypass valve and a series circuit consisting of two more braking resistors, in the order given. The last resistors mentioned can be shorted by contactors. By pulsing the DC control element, the effective braking resistance is changed continuously. A series circuit consisting of a switch, e.g., a switching contactor, and an ohmic field weakening resistor is arranged in parallel to the field winding. By closing this field weakening switch, the motor field may be shunted when the speed of the trolley vehicle is high. This makes the field current smaller than the armature current (field weakening operation) and can prevent an excessively large torque at high speed when the brake is set. In the field weakening region, part of the field current is carried by the field weakening resistor, which is connected parallel to the field winding. In this circuit the field weakening resistor is connected and disconnected by means of the switch in dependence on the speed. The disadvantage here is that the mechanical switch used as the field weakening switch is subject to a certain amount of wear. It is a further disadvantage that, due to the resistance switching, the field weakening only increases or decreases in steps.
It is desirable for the setting of the mutually independent armature and field currents for braking a series DC motor that the field weakening be accomplished gradually and without mechanical switches. The desired braking circuit should be usable not only for pure resistance braking operation or for pure regenerative braking operation, but also for mixed regenerative and resistance braking operation.
The journal "E and M", vol. 85 (1968), no. 3, pages 110 to 117, especially FIG. 8e with associated text, describes a braking circuit for a DC machine using the principle of regenerative braking with automatic field weakening. Here, the armature winding of a series DC machine is shunted by a series circuit consisting of a DC control element and the field winding. The field winding is shunted by a bypass diode which carries the field current if the DC control element is shut off. The armature winding continues to be connected to a load circuit via a return diode.
This prior art braking circuit has the disadvantage that the field weakening sets in automatically only if the DC control element is driven near zero; field weakening is not possible in the rest of the control range.
German Offenlegungsschrift No. 23 29 146 describes a braking circuit for mixed regenerative and resistance braking operation of a series DC machine operating as a generator. The armature and field winding are combined here, preferably in a series circuit, by a stabilizing resistor. This series circuit is shunted by a DC control element as well as by a braking branch, consisting of a braking resistor and a controlled braking valve in series. The series circuit is further connected, via a return diode, to a DC source which can absorb current at least during some of the time. The braking circuit is intended particularly for the series DC machine of a propulsion vehicle which is supplied from a DC trolley wire system. Field weakening operation is not provided in this braking circuit.
It is an object of the invention to provide a combined speed control and/or braking circuit of the type described above in which no overvoltage is produced at the DC control element at the instant when the control element is opened. In addition, a new method for operating the travel circuit is provided.